Until you realise that your life is important and full of opportunities, you will not value your life/time on this earth.
Until you learn to value your life/time on this earth, you will not be able to live life to its full potential.
Does this interpretation help? Perhaps I have just created more confusion!
Also, you should use can't in your thread title.
I couldn't understand this quote suggests that at a time in the past you weren't able to understand it, but now you do. Could is the past simple of can.
I am not a teacher.
Student or Learner